Catherine Massart1, Françoise Gasser2 & Arnaud Agin2,3

Introduction: Highly sensitive TSH assay is required for thyroid exploration and free thyroxine (T4) concentrations should be measured in some cases of thyroid dysfunction.

Objective: To study the analytical performances and reference ranges of new ultrasensitive TSH (TSH3-UL) and new free T4 (FT4) assays performed on the ADVIA Centaur® Immunoassay Systems (Siemens Healthcare Diagnostics).

Patients and methods: We compared the values of TSH and free T4 measured in 251 sera on the ADVIA Centaur® Systems with the previous (TSH-3 and FT4) and the new (TSH3-UL and FT4) assays. Reference intervals were obtained from 270 blood donors (128 women and 142 men, 2061 years old) from the Western (n=137) or the Eastern (n=133) regions of France. They were euthyroid, without any medical treatment and negative for the research of anti-thyroperoxidase antibodies (DYNOTest anti-TPOn RIA, Brahms or Access® TPOAb, Beckman Coulter). Regression linear test and Bland-Altman method were used for comparison of results.

Results: Intra- and inter-assay variabilities were correct with CVs ≤5% or ≤7.2% for TSH3-UL or FT4, respectively. The functional sensitivity of the TSH3-UL assay performed on 49 series during a 8 months period was 0.012 mIU/l. Dilution tests carried out on five sera for TSH3-UL assay gave straight lines. Good agreement and excellent correlations were found between the previous and the new assays (r=0.98 and 0.92 for the TSH and the free T4 assays, respectively). The reference range for TSH3-UL performed after logarithmic transformation of values was 0.633.4 (median: 1.5) mIU/l. For FT4, the reference range was found at 10.317.6 (median:14.3) pmol/l.

Conclusion: The new TSH3-UL shows a functional sensitivity in agreement with NACB guidelines. The reference ranges obtained in a large population of healthy subjects can be used for interpretation of TSH and free T4 results in exploration of thyroid.